Nitride semiconductor devices are characteristic in terms of their higher saturation electron velocities and wider band gaps. Efforts for developing devices with higher withstanding voltages and higher outputs by exploiting such characteristics have been made actively.
Particularly, one type of nitride semiconductor devices used for such devices with higher withstanding voltages and higher outputs is a field effect transistor, particularly, a high electron mobility transistor (HEMT).
For example, there exists a GaN-HEMT including a HEMT structure, where an AlGaN electron supply layer is stacked on a GaN electron transit layer. In a GaN-HEMT, strain is generated in AlGaN due to the difference in the lattice constants of AlGaN and GaN, which induces piezo polarization. Higher-density two-dimensional electron gas (2DEG) is generated by the piezo polarization and the spontaneous polarization of AlGaN. In this manner, a GaN-HEMT can provide a device with a higher withstanding voltage and a higher output.
Various techniques have also been developed to achieve normally-off operations in a GaN-HEMT which provides higher-density 2DEG.
For example, one technique achieves normally-off operations by etching an electron supply layer right under a gate electrode. Hereinafter, this technique is referred to as a first technique.
Alternatively, (intentionally injects carriers) there are devices which operate based on a principle totally different from junction field-effect transistors (JFETs) in the point that carriers are intentionally injected. Specifically, another technique in the devices achieves normally-off operations by providing a semiconductor layer with p-type conductivity only right under a gate electrode. Hereinafter, this technique is referred to as a second technique. A further technique provides a nitride semiconductor layer including a region with p-type conductivity right under a gate electrode, while providing high-resistance regions in remaining regions. In this technique, a hydrogen barrier film or hydrogen diffusion film is provided on that higher-resistance region. Hereinafter, this technique is referred to as a third technique.